Interim Influenza Vaccine Effectiveness Against Laboratory-Confirmed Influenza — California, October 2023–January 2024

Surveillance data can provide rapid, within-season influenza vaccine effectiveness (VE) estimates to guide public health recommendations. Mandatory reporting of influenza vaccine administration to California's immunization information registry began January 1, 2023, and mandatory reporting of all influenza laboratory test results, including negative results, was instituted in California on June 15, 2023. These data, collected by the California Department of Public Health during October 1, 2023-January 31, 2024, were used to calculate interim influenza VE against laboratory-confirmed influenza by comparing the odds of vaccination among case-patients (persons who received a positive influenza laboratory test result) and control patients (those who received a negative influenza laboratory test result). VE was calculated as 1 - adjusted odds ratio using mixed-effects logistic regression, with age, race, and ethnicity as fixed effects and specimen collection week and county as random effects. Overall, during October 1, 2023-January 31, 2024, estimated VE was 45% among persons aged ≥6 months, 56% among children and adolescents aged 6 months-17 years, 48% among adults aged 18-49 years, 36% among those aged 50-64 years, and 30% among those aged ≥65 years. Consistent with some previous influenza seasons, influenza vaccination provided moderate protection against laboratory-confirmed influenza among infants, children, adolescents, and adults. All persons aged ≥6 months without a contraindication to vaccination should receive annual influenza vaccination to reduce influenza illness, severe influenza, and strain on health care resources. Influenza vaccination remains the best way to prevent influenza.


Introduction
Each year in the United States, influenza virus is estimated to cause approximately 9-41 million infections, 140,000-710,000 hospitalizations, and 12,000-52,000 deaths (1).Vaccination protects against illness, hospitalization, and death associated with influenza (2), and annual influenza vaccination is recommended for all persons aged ≥6 months (3).However, effectiveness of seasonal influenza vaccines varies by influenza season, the recipient's age, and other factors (4).Mandatory reporting of administration of all influenza vaccine doses to California's immunization information system (IIS) began January 1, 2023.Positive influenza laboratory test results first became reportable in California on October 1, 2019, and negative results became reportable on June 15, 2023.*The California Department of Public Health (CDPH) used these data sources to estimate early influenza season vaccine effectiveness (VE) against laboratory-confirmed influenza during October 2023-January 2024, including by age and influenza type.

Methods
VE against laboratory-confirmed influenza was estimated using a case-control design by comparing the odds of current season influenza vaccination among persons who received a positive influenza test result (test-positive case-patients) and persons who received a negative influenza test result (test-negative control patients).All persons who received testing for influenza using molecular nucleic acid amplification tests at laboratory, hospital, pharmacy, ambulatory, or community-based testing facilities in California during October 1, 2023-January 31, 2024, were eligible for inclusion.Testing and vaccination records were linked using fuzzy matching † ; a person who received two influenza laboratory results ≥1 day apart was considered to have had repeat testing.The earliest positive test result was used to identify influenza casepatients, and the earliest negative test result (among persons who never received a positive test result) was used to identify control patients.Results from laboratories that reported predominantly positive influenza test results (≥50%) on a weekly basis were excluded because negative results were considered likely to be underreported.These excluded results represented approximately 5% of total laboratory reports.A person was considered vaccinated if immunization records from California's IIS documented receipt of ≥1 dose of seasonal influenza vaccine ≥14 days before testing during August 1, 2023-January 31, 2024.Persons who were vaccinated <13 days before the test date were excluded.VE against laboratory-confirmed influenza was calculated as 100% × (1 − adjusted odds ratio [aOR]), where the aOR is the odds ratio of vaccination among test-positive case-patients compared with Overall, adjusted VE was 45% against receiving either a positive influenza A or B test result, 42% against receiving a positive influenza A test result, and 76% against receiving a positive influenza B test result (Table 2).VE exhibited an age gradient and was highest among persons aged <18 years (56%), 48% among adults aged 18-49 years, 36% among those aged 50-64 years, and was lowest among adults aged ≥65 years (30%).An age gradient was noted for both influenza A and B, although the VE for persons aged ≥65 years for influenza B (54%) was notably higher than for influenza A (29%).

Discussion
Analysis of state-level California surveillance data from influenza vaccination and laboratory reporting systems indicates that influenza vaccination provided moderate protection against laboratory-confirmed influenza across all age groups during October 2023-January 2024.Influenza immunization has been demonstrated to avert complications and severe outcomes associated with influenza, including illness, hospitalization, and death (2,5); therefore, annual influenza vaccination is recommended for all persons aged ≥6 months (3).Measured influenza VE in this analysis was highest among children and adolescents and lowest among older adults, a finding that has been observed in previous seasons (6).This finding suggests that influenza vaccination for adults aged ≥50 years is less effective than among other age groups.Vaccination of adults aged ≥50 years remains a high priority given their increased risk for severe influenza, even if estimated VE is lower for older adults compared with younger persons (6).
Influenza vaccination and laboratory reporting requirements allowed for early season California VE estimates using routine surveillance data.Although the outcome of interest in this analysis (positive influenza laboratory test result) is not one used by CDC VE platforms, the results of this study can be interpreted alongside those from established platforms to broaden characterization of VE.In addition, these VE estimates are consistent with current CDC VE estimates and previous season VE calculations of 40%-60% when influenza viruses are well matched to influenza vaccine components.**Integrated community influenza surveillance and immunization data were used to calculate early season VE estimates during the current (2023-24) influenza season in Canada, and this approach might be feasible for jurisdictions that have similar availability of immunization and laboratory surveillance data (7).
In-season early VE estimates that reflect protection against any positive influenza test result can complement established methods that estimate VE against other outcomes or in other settings (8).For example, during a year when mismatched influenza viruses predominate, knowledge of low VE might support communication surrounding additional protective and treatment measures against influenza (e.g., social distancing, promoting proper cleaning and disinfection procedures, and use of antivirals) when indicated.Preparations in response to low VE might be especially important in hospitals and long-term care facilities, including measures such as active Abbreviation: VE = vaccine effectiveness.* Adjusted for age (using natural cubic spline interpolation), race, and ethnicity as fixed effects and specimen collection week and county of residence as random effects, using mixed effects logistic regression.† VE for unknown influenza types was not calculated because of small sample size, and unknown influenza type results were excluded from overall influenza A and B VE estimation.

dose, adjuvanted, or recombinant vaccine, among those aged ≥65 yrs †
Using mixed-effects logistic regression, estimates were adjusted for age, race, and ethnicity as fixed effects; specimen collection week and county of residence were random effects.Separate analyses were conducted to estimate VE by influenza type (A or B) and by age group.All analyses were performed using R software (version 4.3.1;R Foundation).This activity was reviewed by CDPH and CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.§ influenza laboratory test results meeting study inclusion criteria were reported to CDPH, including § 45 C.F.R. part 46, 21 C.F.R. part 56; 42 U.S.C.Sect.241(d); 5 U.S.C.Sect.552a; 44 U.S.C.Sect.3501 et seq.
* Persons of Hispanic or Latino (Hispanic) origin might be of any race but are categorized as Hispanic; all racial groups are non-Hispanic.†CDC's Advisory Committee on Immunization Practices recommends that adults aged ≥65 years receive a high-dose, adjuvanted, or recombinant influenza vaccine.https://dx.doi.org/10.15585/mmwr.rr7202a1test-negativecontrol patients, adjusted for potential confounders.¶ (3).¶ CDC's Advisory Committee on Immunization Practices recommends that adults aged ≥65 years receive a high-dose, adjuvanted, or recombinant influenza vaccine.